The present invention relates to a so-called foil element for electric heating. By foil element is meant a type of element where the resistance material is placed between insulating plastic foils. The plastic foil can comprise a type of silicone, PVC or polyester, and even combinations of these materials often occur. The electric resistance heating member consisting of metallic material may comprise a meander-shaped coil which can be produced by etching of metal foil. The coils can also be produced by punching according to Swedish Patent Application No. 8404231-6. In use, the foil element is placed against the surface of the object to be heated, such as a water mattress.
For many uses of a foil element a means of controlling the temperature on the heated object within a given interval is required. To be able to do this the temperature of the heated object should be sensed in a suitable way. This could be done either by means of a device separate from the foil element or by a device enclosed in the foil element. In both cases the problem of a correct temperature regulation arises. When using a heat-sensitive control device which is separate from the foil element, i.e., is suitably placed at a distance from the foil element to prevent disturbance from the direct heat transmission therefrom, a problem results from a time delay of heat transfer from the element to the temperature sensing device via the object being heated. The delay is a function, among other things, of the rate and conditions of heat transfer and heat conduction and the effect of the element.
If, on the other hand, the control device is mounted in the foil element a reduced time delay is achieved by the vicinity of the heat source to the sensing device. However, other problems occur because of direct heat transmission from the element to the temperature sensor, whereby the control device is heated substantially only by the resistance coil rather than by the object being heated. In one known arrangement, a sensing coil having a surface area of from about 1 to 5 percent of the total surface area of the heating and sensing coils is positioned so as to be encompassed by the heating coil, i.,e., is within the boundary defined by the heating coil. Thus, the heating of the sensing coil is influenced significantly by a direct heat flow from the heating coil, rather than by the more desirable indirect flow from the object being heated. Also, the sensing coil is formed of a different material and of different thickness than the heating coil and thus is not economical from a production cost standpoint. The cost of a temperature sensor and control equipment can in certain cases be significant in relation to the overall cost of the foil element itself. This is, to no small extent, due to the costs arising in connection with the installation of the temperature sensor in the element.
The aim of this invention is to provide a foil element which incorporates therein a control device by means of which the temperature of the heated object can be measured. A further aim of the invention is to reduce the overall costs of the apparatus for temperature measuring by making this control device a part of the foil element.